It is well known to those skilled in the art of drilling fluid design that the exposure to water of some formations or strata penetrated during the excavation of a well may result in an unstable condition in the wellbore. Typically this instability is characterised by sloughing by these water-sensitive formations and sometimes sloughing by formations above and below the water-sensitive strata as well as other factors including mechanically induced instability.
Typically these water-sensitive formations absorb or adsorb water and then expand or “creep” into the wellbore. That is, in this context, creep is understood to be a gradual deformation of the formation which may be considered as similar to the expansion of salt when exposed to water. In the result, creep can result in sticking of the drilling pipes/bit/tools and if it is severe enough, can result in catastrophic wellbore failure.
Many different approaches have been used to attempt to stabilize formations sensitive to water by modification of water-based drilling fluids, by introducing metal halides, polymers, fatty acids soaps, alcohols, asphalts, gilsonite, huminate, surfactants, and other additives to these fluids. Very often, these modifications have minimized the above creep problems and improved the drilling success.
The most successful drilling fluids are typically formulated as water in oil emulsions typically known as invert emulsions or oil-based drilling fluids. In these drilling fluids, water is completely emulsified or dispersed within the continuous oil phase. As is also known, in order to enhance wellbore stability, it is usually the practice to dissolve calcium chloride into the water phase so as to utilize the osmotic force of the dispersed brine phase to balance or pull water from the formation (typically shales or clays) that have been penetrated with the drilling fluid during the drilling process. This dehydration tends to consolidate the formations and reduces or eliminates the possibility of wellbore destabilization due to chemical forces.
Moreover, as is known, these invert emulsion drilling fluids are expensive with typical costs of $1000/m3 for many fluids.
Furthermore, during the well bore excavation process, oil based drilling fluids are “lost” to the borehole due to a number of factors including seepage due to fractures and the porosity of the formation, the hydrostatic pressure of the drilling fluid within the well bore and surface losses as drilling fluid is processed through surface machines such as screening machines, hydrocyclones, and centrifuges. These losses will often account for as much as 75% of the total cost of drilling fluid within a drilling program.
Accordingly, there has been a need to minimize drilling fluid costs, and more specifically, there has been a need for a lower cost drilling fluid which has the same ability to emulsify water as oil based invert drilling fluids. Still further, there has been a need for drilling fluids that are effective in water-sensitive formations such as clays while as known to those skilled in the art, have other desired drilling fluid properties such as being capable of supporting industry standard weighting agents such as barite, barium sulphate, or calcium carbonate as well as an operating temperature range up to 250° F.